1. Field of the Invention
The present invention relates generally to a code division multiple access (CDMA) system, and in particular, to a method for automatically recovering a dropped call in a mobile station and indicating a recovering situation of the dropped call.
2. Description of the Related Art
In general, the operation of a mobile station in a CDMA system can be divided into a main control task, a search task and a user interface task. The main control task initiates every task included in the mobile station and controls operation of each task. The search task searches pilot channels, assigns fingers, and manages sets of active sectors, candidate sectors and neighbor sectors. The user interface task receives function key data input by the user and displays an operating state of the mobile station.
Meanwhile, upon power-on, the mobile station should acquire synchronization of a received pilot signal to properly demodulate received data through channel decoding and deinterleaving. Therefore, the mobile station searches pilot signals received from several base stations through a radio channel, performs initial synchronization for synchronizing a PN phase of the mobile station to a phase of the base station""s having the highest energy, and continuously detects a multipath signal having energy above a specified level for a soft handoff in the corresponding state. Here, the initial synchronization and multipath signal detection are performed by a searcher included in the mobile station under the control of the search task.
In such a CDMA system, the mobile station disconnects a communication link, when the channel environment deteriorates abruptly, considerable attenuation of the signal resulting in the loss of signal reception occurs, or when a call is dropped due to abnormal power control or the control is unavailable due to an abnormal handoff condition. That is, when a call is dropped (or the communication channel is disconnected), the mobile station of the conventional CDMA system forcedly releases the call without recovering the dropped call.
FIG. 1 shows a procedure for processing a dropped call in the conventional main control task. FIG. 2 shows a state transition diagram of the conventional search task.
In FIG. 2, the search task operates under the control of the main control task. As illustrated, when the search task succeeds in acquisition of a pilot signal having the highest energy in a pilot acquisition state 230, the search task transitions to a sync channel state 240 to receive a sync channel message from a base station (referred to hereinafter as a service base station) corresponding to the pilot signal. Further, the mobile station demodulates the sync channel message to acquire a system time, a long code state, a paging channel data rate and other parameters. After successful demodulation of the sync channel message, the search task transitions to a paging channel state 260. The search task then monitors a paging channel messages in the paging channel state to examine whether a paging message is received or not, and demodulates the paging channel signal to acquire system parameter information such as environment information of the target base station and the mobile station. Here, if the mobile station is assigned to operate in a dedicated time slot, the search task receives only the specific slot assigned thereto and transitions to an idle sleep state 60 to conserve power in the mobile station during other time slots.
Meanwhile, the search task manages various information about the service base station, in which the mobile station is presently registered in the paging channel state 260, and the neighbor base stations. The information includes information about the service base station (i.e., active sector) for transmitting the present paging channel message, information about the neighbor sector which may be used for a handoff, information about pilot channel energy transmitted from the neighbor base stations and information about a PN offset of each base station. The pilot channel energy level is used as a reference value for determining whether to perform a handoff.
In addition, the search task""s energy measurement for active sectors, candidate sectors and neighbor sectors, and storage of the measured energy are performed, even during a call of the mobile station, i.e., in a traffic channel state 270. This is because it is necessary to perform continuous energy measurement and management on several base stations to secure a handoff without dropping the call and to dynamically cope with external channel environments.
The mobile station periodically stays in the paging channel state 260 to secure pilot offset information about the neighbor base stations, such as a neighbor list, through demodulation of the paging channel message. Further, during a call, the mobile station secures pilot offset information about the neighbor base stations through the traffic channel message.
Meanwhile, when a paging message is received from the service base station or the mobile station user originates a call during the paging channel state 260, the mobile station and the service base station establish a communication link and exchange various data. When such a call is released normally, the search task selects a base station having the highest energy out of the active, candidate and neighbor sectors managed in the traffic channel state 270 as a reference active sector, and t hen transitions to a sync channel state 240 via an unslew state 280, during which the search task selects the base station having the highest energy. Further, after receiving a sync channel message from the base station corresponding to the selected reference active sector, the search task stays in the paging channel state 260 to conserve power until a paging message is received or a call is originated.
During the call, it is important that the base station signals having sufficiently high energy should be assigned continuously and frequently to the hardware fingers included in the mobile station. In this way, it is possible to maintain a high-quality call to prevent dropping of the call. However, when there is considerable signal attenuation, or it is impossible to receive a signal due to abrupt deterioration of channel environments, data demodulation cannot be performed normally. Therefore, when bad frames are received continuously over a predetermined time period, the mobile station conventionally abandons data demodulation and releases the communication link. That is, when bad frames are received continuously over a predetermined time period, the conventional mobile station ceases data demodulation, assuming that the call is dropped. In other words, when an abnormal call drop is detected during a conversation state 40 of FIG. 1, the main control task of the mobile station transitions from a release substate 45 to a system determination substate 11 as shown in FIG. 1. Further, the mobile station performs initial pilot signal acquisition again in the pilot acquisition state 230 of FIG. 2. Therefore, conventionally, when the call is dropped, there is no way of effectively and automatically recovering the dropped call, so that the mobile station releases the call-dropped communication link and performs the initial pilot acquisition again. In addition, the call is released, unless the user retries call origination.
As stated above, when the communication channel is released, the mobile station of the conventional CDMA system forcedly releases the call rather than recovering the released channel, requiring the user to retry call origination in order to re-establish the released communication link. This presents an inconvenience to the user. In addition, the mobile station of the conventional CDMA system requires a relatively long time to establish initial pilot acquisition in the pilot acquisition state 230 before retrying call origination. Further, for the user to retry call origination over an access channel, the mobile station should detect a PN offset of a base station by demodulating a sync channel message, and detect various system parameters by demodulating a paging channel message.
As described above, when the user retries call origination after the call is abnormally released, the mobile station requires at least 4 or 5 seconds to establish initial pilot acquisition in the pilot acquisition state 230, and additionally, demodulation of a sync channel message and a paging channel message is required. Therefore, in the conventional CDMA system, when the call is abnormally released, the mobile station performs several unnecessary operations and the user is inconveniently required to redial and wait for a long time before the call is successfully re-established.
A more detailed description will now be made regarding how the mobile station operates when the call is released normally in a CDMA system, to help illustrate the difference between the present invention and the prior art.
When a call is normally released, the search task included in the mobile station selects one sector having the highest energy value out of the presently managed active, candidate and neighbor sectors. Thereafter, the search task slews the system time to a PN offset time of the base station corresponding to the selected sector. These actions are done in slew state. This is a previous operation of a multipath search for demodulation of a sync channel from the base station corresponding to the selected sector after the search task enters the sync channel state.
Further, if the mobile station informs the user of a call recovering state in a predetermined method while automatically recovering the dropped call, the user can be easily aware that the dropped call is presently reconnected. This eliminates the requirement for the user to redial, since the mobile station automatically re-established the call.
It is, therefore, an object of the present invention to provide a method for automatically recovering a call which is abnormally released, and re-establishing a dropped call in a mobile station in a CDMA system.
It is another object of the present invention to provide a method for automatically reconnecting a lost communication link effectively and rapidly without requiring the user to retry call origination after an abnormal call drop, in a mobile station in a CDMA system.
It is yet another object of the present invention to provide a method for performing, when a call is abnormally released, energy measurement on PN offset values of neighbor base stations, received from a previous service base station, determining a reference active sector, and automatically performing call re-establishment in a sync channel state or paging channel state according to whether the determined sector is identical to a previous reference active sector, so that the user is not required to redial and wait for a relatively long time before call re-establishment is made successfully, and the mobile station does not perform the conventional unnecessary initial pilot acquisition, thereby reducing the time required to recover the dropped call, and increasing the convenience of the user.
In accordance with one aspect of the present invention, A method for automatically recovering a call in a mobile station for a CDMA (Code Division Multiple Access) mobile communication system, the method comprising the steps of upon detection of an unintended dropped call, performing energy measurement on pilot signals of neighbor base stations and determining a base station according to the energy measurement, said pilot signals being received from a previous service base station; slewing, when the determined base station is not identical to the previous service base station, a PN offset timing position of the mobile station to a PN offset timing position corresponding to the determined base station; obtaining the system information of the determined base station by demodulating a sync channel message and a paging channel message from the determined base station; and performing automatic redialing on the dropped call.
In accordance with another aspect of the present invention, A method for automatically recovering a call in a mobile station in a CDMA mobile communication system, the method comprising the steps of upon detection of an unintended dropped call, transitioning from a traffic channel state to an unslew state; performing energy measurements on pilot signals received from previous service base station and neighbor base stations; determining a base station according to the energy measurement; slewing, when the determined base station is identical to the previous service base station, a PN offset timing position of the mobile station to a PN offset timing position of the previous service base station, and when the determined base station is not identical to the previous service base station, a PN offset timing position of the mobile station corresponding to the determined base station; state transitioning to a paging channel state and demodulating the paging channel message from the previous service base station; and performing automatic redialing on the dropped call.